Fracture Toughness and Subcritical Crack Growth in CVD Diamond

The fracture toughness, stress corrosion and cyclic fatigue properties of polycrystalline chemical vapor deposited (CVD) diamond have been investigated on thick (˜100 to 300 μm) free-standing films. Specifically, the fracture toughness, K _c, of diamond was determined using indentation methods and for the first time by the tensile testing of pre-notched fracture-mechanics type compact-tension samples. Measured K _c values were found to be between 5 and 7 MPa-m^1/2 by either method and to be apparently independent of grain size and shape. Studies on subcritical crack growth (i.e., at stress intensities less than K _c) indicated that CVD diamond is essentially immune to stress-corrosion cracking under sustained loads in room air, water and acid environments. Corresponding experiments to examine susceptibility to cyclic fatigue are currently being performed using indentation-precracked cantilever beams cycled in three-point bending.